Many different
types of equipment have come our way in the past twenty years that have made
marine aquarium keeping easier and more enjoyable, and in my opinion the trickle
filter has had the greatest impact. Even though there was isolated use of
this type equipment in the 1970’s (deGraaf 1973; Moe 1973), it wasn’t until the
mid 1980’s the benefits of the equipment caught the attention of the average
marine aquarist. When George Smit’s articles in 1986 appeared in
Freshwater And Marine Aquariums (FAMA) they noted the benefits of trickle
filtration and the “reef keeping" portion of the hobby was born and the quest
for maintaining various types of corals went into high gear. Because we have
learned that the combination of live rock and a protein skimmer is superior for
popular reef system, trickle filters are no longer as popular as they use to be.
However, they are still extremely useful on many marine systems, especially
heavily stocked fish-only systems and commercial holding systems.

In Part I of this
article I’ll discuss its general function, then how water gets from the aquarium
to its trickle section. Then how it is distributed at the top of the
trickle section and the media it flows over. In Part II, another form of
trickle media is discussed along with its sump section and features one should
look for when purchasing this type equipment.

Trickle Filter
Function

The trickle
filter is usually composed of three separate units: the prefilter, trickle
section, and sump. Aquarium surface water first flows into a skimmer box
or prefilter unit hanging on the side of the aquarium. The mechanically
filtered water then flows via gravity into the top portion of the trickle filter
unit, usually located in a space under the aquarium. When there, it enters
some sort of distribution system, most often a stationary sprinkler pipe and is
distributed over the surface of a drilled plate. This “drip” plate is
usually covered with some sort of mechanical filter material.

Water drips
through the plate and enters the below trickle section where it flows over and
between packing media that fills most of this section of the unit. This
section is usually referred to as the “dry” portion of the equipment because the
packing media is not submerged. The majority of the biological filtration
and gas exchange in this type equipment occurs in this section. The
trickling of water in this so-called “dry” section provides oxygen to nitrifying
bacteria living on the surface of its packing media. This results in an
effluent that is high in dissolved oxygen and nitrate.

When the water
exits the bottom of the trickle/dry section it flows into a large open container
referred to as the sump or “wet” section. This portion of the equipment
may be large enough to hold other filtering aids and a pump to return the water
to the aquarium.

All in all,
trickle filters allow for an unencumbered aquarium interior, free of various
stand pipes, corner filters and air hoses. In doing so it helps provide a
more natural looking environment and increased dissolved oxygen which in turn
helps promote a stress reducing and healthier environment for fishes and
invertebrates.

Siphon/Prefilter

As for its
individual sections, aquarium water first flows into the siphon/prefilter,
sometimes called a skimmer box, where the water from the aquarium begins its
journey to the equipment below the aquarium. Even though this box can be
thought of as a prefilter, its real purpose is to transport “surface” water from
inside the aquarium to the top of the trickle section located below the
aquarium. These units are mainly designed as a siphon box that sit on the
inside and outside top edge of the aquarium. It also functions as a
surface skimmer and can serve as a prefilter.

It benefits the
aquarium in three ways. It first removes surface scum allowing for a
better air/water interface resulting in a better gas exchange at the waters'
surface. Second, it allows for upward flows in the aquarium that helps
bring surfactant compounds towards the surface, improving the chances they will
ultimately be removed by the trickle section or a protein skimmer if so
equipped. Thirdly, the upward flow of water is replaced by a downward flow
of oxygen rich surface water to the animals near the aquarium bottom.

As aquarium
surface water flows into the inside portion of the siphon box it is withdrawn
from this area by a U-shaped siphon tube. It is then transported over the
edge of the aquarium and into its back section. Water in this area then
drains through a mechanical filter, usually a sponge filter, and flows via
gravity downward through a connecting hose to the top inlet of the main trickle
filter unit.

Some siphon boxes
have the means to control aquarium water level and automatically remove any air
trapped in its siphon. Less expensive units simply keep pace with the
volume of water being returned by the trickle filter’s sump pump.

Keep in-mind that
the diameter of the siphon tube and that of the connecting hose must be large
enough to keep pace with the volume of water returned to the aquarium from the
sump pump. If more water is returned to the aquarium then what can be
siphoned out, the aquarium may overflow. Of course, this depends upon the
volume of water in the sump. Siphons also have a way of collecting air at
the top of their curved tube. If the air bubble gets too large, the flow
through the siphon tube will be reduced or stopped.

A short piece of
airline tubing inserted through the inlet side of the siphon until its end
reaches the air bubble can be used to suck air bubbles out. Definitely not
my favorite way of correcting this problem! A much better way is by simply
connecting the tube end to the small air inlet on top of a powerhead which
in-turn will automatically suck out the bubble. In fact, I’ve even seen
some siphon tubes having small air release valves built into their curved
section. When opened, it allows the air bubble to escape. The “C”
Siphon from CPR automatically controls aquarium water level, muffles the sound
of draining water, and removes any accumulating air in its siphon. It’s my
choice when it comes to an almost maintenance-free siphon box.

Keeping the
mechanical filter media in siphon boxes clean is also important. Dirty
filter media can restrict water flow to the trickle section below and cause an
overflow condition in the aquarium. Some siphon units use a section of
hollow sponge that surrounds its drain connection. When the sponge becomes
clogged, water simply overflows the open top edge of the sponge and flows into
the drain line.

The sound of
water entering the drain tube is sometimes quite noisy. By placing a short
standpipe drilled with many holes, water flow into the drain tube will become
somewhat quieter. All in all, prefilters are fairly simple devices and
require little maintenance.

Unfortunately
some trickle filters come with inadequate or no siphons/prefilters simply
because the maker wants to save a few dollars so as to make their unit look like
a better deal. If there is no siphon/prefilter, beware! Check out
the cost of one and add it to the cost of the unit in question. Then
compare total cost to another brand that sells complete units. Make sure a
separately purchased unit will handle sump pump water flow. If the trickle
filter of your choice has a siphon/prefilter that looks kind of lacking in
design, I suggest looking for another brand.

Aquariums can
also be purchased in "reef ready" configurations or can be drilled and fitted
with bulkhead fittings that allow them to drain surface skimmed water to a sump
without the need for siphon overflows. Many aquarists prefer these options
for their "cleaner" look and reduced risk of flooding the tank.

Trickle Section

Way back when
this type equipment first came to market, water from the aquarium would first
flow into a single or combination of stationary PVC pipes that contained many
holes drilled throughout their length. Water flowed out these holes and
‘hopefully’ evenly covered the top drip plate of the trickle section. Then
came the tinkers who felt a better way was needed to distribute the water over
the top of the drip plate and ‘invented’ the rotating sprinkler pipe. But
that not only added additional product cost, it led to much controversy.

Many aquarists
reported that when plastic ball type packing was utilized some rotating
sprinkler bars did not throw the water far enough to reach into the corners of
the square-shaped trickle chamber. Therefore the packing in corner areas
never become wet and failed to become colonized with nitrifying bacteria,
defeating a portion of the system. In some cases they rotated too fast and
threw the majority of the water to the sides of the container. This
allowed most of the water to run down the insides of the trickle section never
wetting the central packing media. And even if a roll of double layer
spiral (DLS) material was used instead of bio-balls, some water dripped down in
corner areas where it couldn’t come in contact with the roll of DLS.
Besides deficiencies in rotating mechanisms and ongoing debates about water
distribution, the added product cost brought an end to rotating sprinkler pipes.
The less expensive way to distribute water won out, and besides, was more
dependable and easier to maintain

Trickle Section
Packing (Bio-Balls)

The most
important factors as to trickle filter efficiency are the amount of surface area
on the packing media below the drip plate and the even distribution of water
when flowing over it.

The most popular
form of trickle section media has been the plastic, usually ball-shaped items
that got their start in air and waste treatment facilities. Most of these
media, properly called “packing,” were engineered to remove pollutants from
smokestacks or sewage treatment plants. Since this packing can provide
surface area for the colonization of nitrifying bacteria, even those designed
for smokestacks, they have progressed from their original function into the
aquarium trade. When used in aquarium trickle filters, the ever-moving
thin film of water over the packing surface provides an excellent gas exchange
area with the oxygen used by nitrifying bacteria living on their surface quickly
replenished. And, since these packing don’t wear out they are very cost
efficient and need little or no maintenance. Therefore, nitrifying
bacteria living on this media receive all the oxygen they need and because of
the trickling effect, highly oxygenated water flows back into the aquarium.

Of course this is
not the case in the UGF system or the fluidized bed filter, where oxygen is
removed by bacteria living on the gravel or sand particle and the water flows
minus much of its oxygen back to the aquarium.

Whether this
plastic material is shaped like a sphere, ring, cube, or whatever, “surface
area” is probably the most important criteria for determining its cost
effectiveness. And surface area is generally measured in square feet on
the total number of individual packing that will fit into a one-gallon
container. What constitutes how many gallons of packing are needed to
support a given size aquarium has always been a good question. You’ll
probably find almost everyone has a different answer.

My rule-of-thumb
is that a gallon of packing containing 10 sq. ft. of ‘useable” surface area is
the minimum needed for every 10 gallons of water in a fish-only aquarium.
This is based upon my personal experience in my fish-only systems.
Depending upon the surface area provided by the brand media of your choice, the
number of gallons required to fit a system must be adjusted. For a heavily
loaded fish-only system I would increase the recommendation to 1.2 gallons of
packing per ten gallons of water. As for their use in a reef aquarium the
10 sq. ft. of surface area should be applied to 20 gallons of water. (But
prefer such equipment not be used on reef aquariums because of the nitrate
content produced.) Usually, the packing manufacturer or distributor will
or should supply that data.

Useable surface
area is that which gets fully wet. That depends upon its physical design.
This ‘wettability’ factor is extremely important, as nitrifying bacteria will
not colonize a dry surface. I don’t know of any chart or rating factors
for "wettability", so I find the human eye to be the best judge. I
personally prefer the architecture of the ball-shaped packing and have used it
in many of my past aquariums.

Another fairly
important criteria is void space, which is the ratio of how much area is
actually occupied by the packing structure and the space for water and airflow
through the individual packing. Void space is only important to the point
of allowing for good distribution of water and airflow. However, there are
some that tout void space as an important ability of their product to degas
ammonia. After many discussions with environmental engineers on this
subject, none believed this possible in the size equipment used on home aquaria.
These same engineers chose ball-shaped packing for their very large commercial
stripping towers because of their excellent "wettability" and water and airflow
characteristics. In fact, all agreed that a structure eight feet high with
a large volume of airflow would be required before even minute amounts of
ammonia could be “degassed.”

And keep in mind
that water and airflow must be evenly distributed throughout the packing that
fills the trickle section. Any type packing that would contribute to
channeling, i.e., the concentration of either water or air in streams instead of
even and sheet-like distribution, would distract from the media cost
effectiveness even if it had greater surface area than other type packing.

There are some
other points to remember when wanting to gain the most efficiency from the
selected packing. The first and most important is the application of an
outside airflow through the packing. By placing one or more airstones
under the column of packing, dissolved oxygen in the aquarium can go from
unacceptable to saturation. Of course, when one does this the top of the
trickle tower should have a vent hole for the additional air to vent. And
make sure all the packing is located above the water level in the sump.
Submerged packing, whether at the bottom of the trickle section or in the sump
only detracts from the dissolved oxygen gained by the water evenly flowing over
the exposed packing in the trickle section.

Trickle Section
Packing (Double Layered Spiral)

Double layer
spiral (DLS) is another form of trickle section packing. It is a white
polyester material sandwiched between layers of black woven plastic. When
new it has greater surface area for the same given space taken up by plastic
ball-type packing. It also allows for good airflow; does not restrict
water flow; distributes water very evenly; allows for greater contact time; and,
is less expensive than plastic ball-type media. The key words here are
“when new.”

Unfortunately DLS
becomes coated with detritus and becomes a mechanical filter instead of a
biological filter. Prefiltering the water helps diminish buildup, but does
not eliminate it on this type media. And because of that, it periodically
requires cleaning. Yet even the most gentle cleaning methods will wash
away much of its nitrifying bacteria since the material itself is very smooth
and bacteria have no firm grip on its surface. It will be weeks before it
becomes recolonized.

The biggest
problem with DLS is that it is used as a “roll” of media. My personal
opinion is the media should be sliced into pads, similar to slices of bread in a
loaf. Then, the entire trickle section could be filled with vertical
“slices” of DLS, which would also eliminate the open corner situation in the
trickle tower. This would allow the removal of one or two slices per month
for cleaning without radically affecting the overall efficiency of the entire
filter as would happen if the entire roll were removed for cleaning.

There are some
other considerations to take into account when using this inexpensive media.
First, the physical shape of the material makes it somewhat inefficient in high
flow areas. Because of water shear, i.e., the force generated by flowing
water, nitrifying bacteria can be washed off its thin smooth string-like
surfaces in high flow areas. Therefore I recommend not using DLS in
systems where there are high flow rates. Also, since DLS appears not to
degas carbon dioxide as effectively as do most plastic ball type media, I prefer
to utilize this media in aquariums where macroalgae will be one of the more
desired inhabitants.

Another factor to
be considered is its initial cost. DLS is very inexpensive when compared
to most plastic ball type media, but will not last indefinitely (you know what I
mean) as will plastic packing. DLS will no doubt have to be replaced in a
few years, increasing overall cost of the system and increasing time spent for
general maintenance.

As to what
constitutes how much of this type product would be needed to support a given
size aquarium is somewhat guesswork on my part. In my opinion, a layer of
DLS that is .5 inch (1.5 cm) thick, 10 inches (15 cm) high and 25 feet (8
meters) long would provide biological filtration for an aquarium up to 100
gallons. Another rule of “my” thumb is 15 cubic inches per gallon.

Keep in mind,
just as it is preferable to maintain a constant upward airflow throughout a
column of plastic ball type media, it is also preferable to supply a flow of
fresh air through a column of DLS. This can be accomplished by simply
placing one or two airstones under the trickle section. My dissolved
oxygen tests have shown that a single plugged airstone can lead to a reduction
of approximately 1.0 ppm of dissolved oxygen in the aquarium. That is
quite a large amount and could be the difference between success or failure in
some systems.

Sump

After water flows downward through the trickle section it falls into the sump or
what is usually referred to as the “wet” portion of the equipment. Some
aquarists are under the impression that by putting gravel or packing in the sump
they can increase the biological capacity of the trickle filter. True, but
not a good idea when you think of all the trapped particulate matter breaking
down into unwanted compounds. If that is not reason enough not to place
gravel or packing in the sump, consider the myriad of bacteria living on the
surface of this material and the amount of oxygen they consume. In the
trickle section the oxygen is being replenished as water flows over and past the
packing. Not so with submerged media! The additional oxygen the
trickle section delivers to the aquarium is one of the main reasons this type
equipment is purchased. Why defeat it by having an unneeded large
community of oxygen consuming bacteria living in the wet section. There are some
important aspects to look for in a quality made wet section. They are
listed in the “Features” section below.

Features

As for some of
the embellishments noted below, probably not all will be found on any single
brand. If they were, some might not be used or even needed and may only
unnecessarily add to its cost. Nevertheless, the aquarist could install
those of interest if fairly handy with some simple tools.

* A large water
reservoir/sump/wet section, because this type equipment encourages high
evaporation.

* An
overflow/prefilter so surface water will be constantly drawn off and filtered
before entering the top of the trickle filter.

* Check to see if
a sump water pump is included or an option/extra cost? Remember that water
flow through the trickle section should flow at about four times the volume of
the aquarium. Therefore a pump that has a flow rate of five to six times
the volume of the aquarium will probably be needed. This extra pumping
capacity is needed because of flow reductions caused by how high the water needs
to be raised above the pump (called “head”) so that it can be returned to the
aquarium. Curves and angles in the water supply tubing/piping also reduce
flow as does wear on pump parts, as they become less effective with age.

* Is there an
instruction manual. Sounds simple enough but you would be surprised what
questions can come up after you have the equipment at home.

* Look for a
built-in connection for air pump tubing at the base of the trickle tower.
Some brands not only have the connection, but a distribution system, i.e.,
piping and airstones under the trickle section.

* Consider having
a short length of vertical plastic pipe built into the drip plate to serve as an
overflow. Should the drip plate’s mechanical filter clog, water could
buildup and overflow the top of the unit. A short length of pipe will also
serve as an airflow tube. It allows the air that may be injected under the
trickle packing to flow to the topside of the drip plate. Note, you might
want to place a vent hole in the top cover of the trickle section to allow the
air to escape so its not forced to go up against the downward flow of water
coming from the aquarium.

* Slide-out, draw
type drip trays are a nice feature and make it easy to replace dirty mechanical
filter media without shutting down and removing the top of the trickle section
unit to perform this maintenance.

* Having a “Low
and High” watermark on the sump side is also a nice feature. These
reference marks will let the aquarist know how close they are to having to add
water to make up for evaporation. It really helps to prevent salinity
swings in the aquarium.

* Some also
contain protein skimmers. Look for a quality brand skimmer!

* Look for an
angled deflection plate under the trickle section. By directing the flow
towards the opposite end of the sump any detritus that may slip through the
trickle section will be deflected to an area in the sump where it is easier to
remove.

* Find out if
there is a real person at the place that manufacturers the unit and who will be
available to answer questions.

* Does the unit
have a manufacturer’s warranty? If made by a reputable company there is no
good reason why they should not stand behind their product.

* Is there a
“weir” in the sump. The weir is a solid plastic divider that basically
separates the sump into two separate sections. One section receives water
from the trickle section. Its water then overflows the divider/weir and
flows into the section containing the return pump. By dividing the sump
into two sections should the prefilter clog, only the water that reaches the
pump section will be returned to the aquarium. The water on the other side
of the weir will remain in the sump. If the sump did not have a weir and
the prefilter clogged, “all” the water in the sump would be returned to the
aquarium and may cause an overflow. Then try to explain the wet carpet to
your spouse.

Summary

Whatever form
this equipment takes, i.e., hang-on-back, a stand-alone unit, or built into the
back wall or side of the aquarium, the trickle or wet/dry filter as many call
it, is still a very useful piece of equipment.

And if so
equipped, it should be located in a dimly lit area so as to prevent algae from
growing and clogging the trickle section. Keep in mind algae growth on the
packing material can cause channeling of the water and defeat the purpose of the
unit which is to provide a means for nitrifying bacteria to carryout their
function.

Nevertheless, because its final product is nitrate, it has fallen into disfavor
with some reef aquarists. I can understand that as keeping the nitrate
level quite low is an important factor when maintaining the more complex reef
aquariums. And since protein skimmers, which almost all reef systems
utilize, increase dissolved oxygen and reduce the nutrients that can be oxidized
into nitrate, the need or usefulness of a trickle filter has become quite
diminished on many reef systems. However, where fish-only and general
invertebrate systems are involved, and where nitrate levels are not of great
importance, trickle filters remain an excellent type of biological filtration
equipment.